An optical fiber (i.e., a glass fiber typically surrounded by one or more coating layers) conventionally includes an optical fiber core, which transmits and/or amplifies an optical signal, and an optical cladding, which confines the optical signal within the core. Accordingly, the refractive index of the core nc is typically greater than the refractive index of the optical cladding ng (i.e., nc>ng).
For optical fibers, the refractive index profile is generally classified according to the graphical appearance of the function that associates the refractive index with the radius of the optical fiber. Conventionally, the distance r to the center of the optical fiber is shown on the x-axis, and the difference between the refractive index (at radius r) and the refractive index of the optical fiber's outer cladding (e.g., an outer optical cladding) is shown on the y-axis. The refractive index profile is referred to as a “step” profile, “trapezoidal” profile, “parabolic” profile, or “triangular” profile for graphs having the respective shapes of a step, a trapezoid, a parabola, or a triangle. These curves are generally representative of the optical fiber's theoretical or set profile. Constraints in the manufacture of the optical fiber, however, may result in a slightly different actual profile.
Generally speaking, two main categories of optical fibers exist: multimode fibers and single-mode fibers. In a multimode optical fiber, for a given wavelength, several optical modes are propagated simultaneously along the optical fiber. In a single-mode optical fiber, the signal propagates in a fundamental LP01 mode that is guided in the optical-fiber core, while the higher order modes (e.g., the LP11 mode) are strongly attenuated.
For the same propagation medium (i.e., in a step-index multimode optical fiber), the different modes have different group delay times. This difference in group delay times results in a time lag (i.e., a delay) between the pulses propagating along different radial offsets of the optical fiber. This delay causes a broadening of the resulting light pulse. Broadening of the light pulse increases the risk of the pulse being superimposed onto a trailing pulse, which reduces the bandwidth (i.e., data rate) supported by the optical fiber. The bandwidth, therefore, is linked to the group delay time of the optical modes propagating in the multimode core of the optical fiber. Thus, to guarantee a broad bandwidth, it is desirable for the group delay times of all the modes to be identical. Stated differently, the intermodal dispersion should be zero, or at least minimized, for a given wavelength.
To reduce intermodal dispersion, the multimode optical fibers used in telecommunications generally have a core with a refractive index that decreases progressively from the center of the optical fiber to its interface with a cladding (i.e., an “alpha” core profile). Such an optical fiber has been used for a number of years, and its characteristics have been described in “Multimode Theory of Graded-Core Fibers” by D. Gloge et al., Bell system Technical Journal 1973, pp. 1563-1578, and summarized in “Comprehensive Theory of Dispersion in Graded-Index Optical Fibers” by G. Yabre, Journal of Lightwave Technology, February 2000, Vol. 18, No. 2, pp. 166-177. Each of the above-referenced articles is hereby incorporated by reference in its entirety.
A graded-index profile (i.e., an alpha-index profile) can be described by a relationship between the refractive index value n and the distance r from the center of the optical fiber according to the following equation:
  n  =            n      max        ⁢                  1        -                  2          ⁢                                    Δ              ⁡                              (                                  r                  a                                )                                      α                              
wherein,
α≧1, and α is a non-dimensional parameter that is indicative of the shape of the index profile;
nmax is the maximum refractive index of the optical fiber's core;
a is the radius of the optical fiber's core; and
  Δ  =            (                        n          max          2                -                  n          min          2                    )              2      max      2      
where nmin is the minimum refractive index of the multimode core, which may correspond to the refractive index of the outer cladding (most often made of silica).
By adjusting the value of the parameter α, it is possible to obtain a group delay time that is virtually equal for all of the modes. Stated differently, the refractive index profile can be modified to reduce or even eliminate intermodal dispersion, thereby increasing bandwidth.
Typically, multimode optical fibers (MMFs) have central core diameters of about 50 microns (i.e., 50-micron MMFS) or 62.5 microns (i.e., 62.5-micron MMFS). For such multimode optical fibers, the parameter α is typically between about 1.9 and 2.2 and can be chosen to provide a large bandwidth at a target operating wavelength (e.g., 850 nanometers or 1300 nanometers).
Generally speaking, high bandwidth and low bending losses are desirable characteristics of multimode optical fibers for multi-gigabit Ethernet communications. One proposed method of achieving reduced bending losses involves adding a depressed trench having a large volume between the core and the cladding. Nevertheless, the position and the depth of the trench can significantly affect the optical fiber's bandwidth
Furthermore, although the depressed trench typically improves the bend resistance of the guided modes, it also allows additional modes, called “leaky modes,” to co-propagate with the desired guided modes.
These leaky modes exhibit additional losses, called “leakage losses.” Typically, wider depressed trenches reduce the leakage losses of the leaky modes. In addition, the deeper the depressed trench (i.e., in terms of absolute value, the bigger the negative refractive index difference of the depressed trench with respect to the outer cladding), the greater the number of leaky modes.
Leaky modes are also present within a regular MMF (i.e., an MMF without any significant improvement in bend resistance), but the existence of the leaky modes is disregarded in practice, because the level of their leakage losses is extremely high.
That said, with conventional trench assistance, the leakage losses of the leaky modes are so reduced that the leaky modes can propagate over several meters and even more, depending on the trench design that is required for compatibility with a regular MMF.
U.S. Patent Application Publication No. 2009/0154888, U.S. Patent Application Publication No. 2008/0166094, Japanese Publication No. 2006-047719, U.S. Patent Application Publication No. 2010/0067858, and U.S. Patent Application Publication No. 2009/169163, each of which is hereby incorporated by reference in its entirety, disclose trench-assisted MMFs. None of these publications, however, discloses the impact of leaky modes.
Commonly assigned French Publication No. 2,949,870 and its counterpart U.S. Patent Publication No. 2011/0058781, each of which is hereby incorporated by reference in its entirety, address the issue of leaky modes. Nevertheless, the publication focuses on the numerical aperture (i.e., on the far field) rather than on the size of the inner core (i.e., the near field) to limit the contribution of the leaky modes.
Thus, there exists a need for a trench-assisted graded-index multimode optical fiber having reduced bending losses that limits the impact of the leaky modes on other optical-fiber characteristics (e.g., core size and numerical aperture).